1. Field of the Invention
The invention relates to generators with current regulated inverters, including wind and water current turbines, and more particularly to variable speed wind turbines employing multi-phase generators using full power conversion systems with utility fault ride through capability.
2. Description of the Prior Art
The application of wind-powered generating systems in the past has been on a small scale when compared to the total generating capacity of the electrical grid. A term which is often used to describe the relative quantity of the wind generated power is “penetration”. Penetration is the ratio of wind generated power to the total available generated power. Even in those states where wind generated power is highest, the penetration levels are under, or about one percent. While this is a relatively small amount of power, and the rules that govern the operation of the turbines reflect this small penetration, it is clear that the operating rules are changing. This is in anticipation of substantially higher penetration levels into the future. One operating principle that is being revised is how a wind turbine reacts to a fault on the utility grid transmission (or sub-transmission) system to which the wind turbine is interconnected. A fault is defined as a utility disturbance that results in voltage on the utility transmission system decreasing by a significant amount for a short duration (typically less than 500 miliseconds). Faults can be caused by at least one phase conductor being inadvertently connected to ground (a ground fault), or the inadvertent connection, or short circuiting of multiple phase conductors. These types of faults occur during lightning and wind storms, or when a transmission line is involved in a vehicle accident, as examples. A significant reduction in voltage can also occur when a large change in electrical load or electrical generation occurs nearby in the utility transmission system. Examples of this type of event could include sudden disconnection of a large power plant, or sudden connection of a large load such as a steel mill. This type of a voltage reduction event is not typically referred to as a fault in utility parlance, although for the purposes of this specification the term “fault” is intended to cover such voltage reduction events. The term “fault” as used herein, is intended to cover any event on the utility system that creates a momentary reduction or increase in voltage on one or more phases. In the past, under these inadvertent fault and large power disturbance circumstances, it has been acceptable and desirable for a wind turbine to trip off line whenever the voltage reduction occurs. Operating in this way has no real detrimental effect on the supply of electricity when penetration is low. This operating rule is under revision however, and it is now desirable for a wind turbine to remain on line and ride through such a low voltage condition. This new operation is similar to the requirements applied to traditional generating sources such as fossil fueled synchronous generator plants. The reason for this requirement is straight forward; if wind generated power is at a high level of penetration, and a momentary fault occurs, the dropping of the significant amount of wind generated power (as required under the old operating rules) can cause much more serious stability problems, such as frequency swings, or large system wide instabilities of generation systems. These are very extensive fault conditions and can lead to the disruption of power to large regions, effecting large numbers of utility customers. Using variable speed wind turbines to generate electrical power has many advantages that include higher blade efficiency than constant speed wind turbines, control of reactive power-VARs and power factor, and mitigation of mechanical turbine drivetrain loads. The low voltage ride through requirement described above, often referred to as utility fault ride through, is also more easily addressed using certain variable speed wind turbine technology as will be disclosed herein. In considering variable speed wind turbines, it is important to examine two classes of power converters which are used and which could be used for the utility ride through function.
One prior art variable speed wind turbine uses a total conversion system to completely rectify the entire power output of the wind turbine. That is, the wind turbine, operating at a variable frequency and variable voltage, converts this power into a fixed frequency and voltage that matches that of the grid. An example of this type of system is disclosed in U.S. Pat. No. 5,083,039 (incorporated herein by reference) which comprises a turbine rotor that drives a pair of AC squirrel cage induction generators with two respective power converters that convert the generator output to a fixed DC voltage level. The DC bus of this system is then coupled to the utility inverter and power is inverted at fixed frequency and supplied back to the utility. The generator control system in U.S. Pat. No. 5,083,039 uses field orientation principles to control torque and uses real and reactive power control methods to control the utility inverter. While generation in this turbine requires only unidirectional power flow, a bidirectional converter is inherently required as the induction generators need to be excited from the DC bus. The DC bus in this system is controlled from the utility inverter portion of the conversion system and control of the DC bus is difficult when the utility voltage falls substantially.
A second example of a total conversion system is that disclosed in above-identified U.S. patent application Ser. No. 10/773,851. This system utilizes synchronous generators together with a passive rectifier and active utility inverter to convert generator variable frequency and voltage to utility compatible frequency and voltage. This system is inherently unidirectional in its ability to pass power from the generator to the grid. An advantage of this system is that the DC bus is controlled from the generator side of the power conversion system and bus control is straight forward during the periods of low utility voltages.
U.S. Pat. Nos. 6,137,187 and 6,420,795 (both incorporated herein by reference) describe a partial conversion, variable speed system for use in wind turbines. The system comprises a wound rotor induction generator, a torque controller and a proportional, integral derivative (PID) pitch controller. The torque controller controls generator torque using field-oriented control and the PID controller performs pitch regulation based on generator rotor speed. Like the U.S. Pat. No. 5,083,039, power flow is bidirectional within the rotor of the generator and an active rectifier is used for the conversion process. The converter used in this system is rated at only a portion of the total turbine rating, with the rating depending on the maximum generator slip desired in the turbine design. The converter controls the current and frequency in the rotor circuit only with a direct electrical connection between the generator stator and the utility. In addition to the converter controlling torque in this system, the converter is capable of controlling system reactive power or power factor. This is accomplished by under/over exciting the generator rotor circuit along its magnetization axis. The converter is connected in parallel to the stator/grid connection and only handles rotor power input and output. This system is difficult to control in the event of a sudden drop in utility voltage. This is because the rotor converter DC bus is controlled from the utility side converter just as in the U.S. Pat. No. 5,083,039 and because the generator stator is directly connected to the utility. The direct stator connection creates problems in that no converter is between the stator and utility and transient currents and torques are generated which are not subject to control by an intervening converter.
U.S. Pat. No. 7,042,110 (incorporated herein by reference) describes a system for regulating a wind turbine connected at the utility distribution level based on the voltage of the system. U.S. Pat. No. 7,042,110 stands in contradistinction to the fact that most wind generation in the United States is connected at the sub-transmission level. Moreover, the method described does not address the sudden, deep drop of utility voltage.
It is desirable to provide a variable speed wind or water current turbine, which has the ability to continue inverter control during a utility fault, such as a sudden, deep drop of utility voltage.
It is also desirable to provide a ride-through capability for a wind or water current turbine system, in which the generator is completely decoupled from a utility grid and its disturbances.